1. Field of the Invention
The present invention is related to an optical transmitter which receives backward light of a laser diode by a monitoring light detector, and is applied particularly to optical communication or optical recording/reading.
2. Description of the Prior Art
The prior art will be described with reference to FIG. 1. FIG. 1 is rough explanatory drawing of an optical transmitter according to a prior art example. In FIG. 1, a laser diode 81 outputs light in the two directions forward and backward. Normally, the forward light 82 is used as output light to the outside of the apparatus, and the backward light 83 is used as output light for monitoring. Because the proportion of output light with respect to the input current changes due to temperature and degradation over time, the backward light is monitored and a negative feedback loop is created to control the output light of the laser diode, whereby it is possible to plan the stabilization of the output light of the optical transmitter.
In the prior art, in order to monitor the backward light of the laser diode, a substrate of a light detector 84 is designed to be transparent to the wavelength of the output light from the laser diode, and the apparatus is constructed with the light detector 84 provided on the same packaging base 86 as the laser diode 81.
However, in the example where InGaAs is used as a light detector, because InGaAs has a large refractive index of 3.56, even when the backward light 83 of the laser diode is incident on the side surface of the light detector 84 facing the laser diode 81, the light will propagate roughly parallel to the semiconductor junction plane 85 inside the light detector after being refracted by such side surface, and the proportion of such backward light 83 absorbed by the depletion layer of the light detector 84 was small. Even when the backward light 83 from the laser diode 81 is reflected by the surface facing such side surface, because the light will propagate roughly parallel to the semiconductor junction plane 85, the proportion of such backward light 83 absorbed by the depletion layer of the light detector 84 is still small.
Further, even though it is possible to consider a method in which the angle formed between the side surface where the backward light of the laser diode is incident and the junction plane of the light detector is made an acute angle so that the backward light 83 from the laser diode 81 is sent to the semiconductor junction plane 85 after being incident from such side surface, because the refractive index of light at the boundary from air to the light detector is used, such side surface needs to have a large inclination angle.
Furthermore, because the light detector 84 excluding the depletion layer is made transparent to the output light from the laser diode, when stray light 87 inside the optical transmitter is incident on the light detector 84 and absorbed by the depletion layer 85, this causes outside interference with the negative feedback loop, and there is the risk that the negative feedback loop for stabilizing the output light of the laser diode will not be carried out normally.